A phosphor is a luminescent material that absorbs radiation energy in one portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. One important class of phosphors includes crystalline inorganic compounds of very high chemical purity and of controlled composition, to which small quantities of other elements, called “activators,” have been added for fluorescent emission. With the right combination of activators and inorganic compounds, the color of the emission of these crystalline phosphors can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic energy outside the visible range. Well known phosphors have been used in mercury vapor discharge lamps to convert the ultraviolet (UV) radiation emitted by the excited mercury to visible light. Other phosphors are capable of emitting visible light upon being excited by electrons, useful in photomultiplier tubes, or X-rays, such as scintillators used in imaging systems.
One important property of phosphors is the decay time, e.g., the time required for the phosphor to stop emitting light after the excitation is removed. Most phosphor compositions have extremely short decay times, with most of the stored energy emitted as light within seconds, or even a small fraction of a second after excitation ends. These phosphors may be useful in lighting type applications where continuous excitation is present. However, in many applications it would be worthwhile to have a phosphorescent material that continues to emit light for long periods of time after excitation has ended.
Persistent phosphors based Sr2MgSi2O7 host lattices and activated with Eu2+ and Dy3+ were developed to provide a blue persistent phosphor that has found wide application in both the aesthetic and functional markets, including, but not limited to entertainment, safety and emergency lighting and in exit signage. However, the applications of these materials are limited mainly due to their initial intensity and length of persistence as these pyrosilicate based materials have a relatively low initial intensity and decay relatively quickly.
Accordingly, it would be beneficial to provide a blue (Sr rich) to yellow (Ca rich) persistent phosphor having a higher initial intensity and slower rate of decay as compared to the prior art pyrosilicate based blue persistent phosphor materials. It would also be beneficial to provide methods of making these blue (Sr rich) to yellow (Ca rich) persistent phosphors as well as articles utilizing such persistent phosphors.